The hydraulic driving apparatus installed on the work machine generally includes a hydraulic pump that discharges hydraulic oil, and a hydraulic actuator that receives the hydraulic oil discharged by the hydraulic pump, and operates so as to move a driving subject, and there has been known a technology of using a so-called pump motor where the hydraulic actuator simultaneously has a pump function and a motor function in order to conversely regenerate an energy by means an external force given by the driving subject in recent years.
For example, Patent Document 1 discloses an apparatus including multiple reversible adjustment units, each of which is a so-called pump motor. This apparatus includes a first reversible adjustment unit E1 and a second reversible adjustment unit E2 provided in a rotation driving circuit that rotates a rotating body, and a third reversible adjustment unit E3 and a fourth reversible adjustment unit E4 provided in a boom driving circuit including a boom cylinder that drives a boom.
The first reversible adjustment unit E1 operates as a pump that discharges hydraulic oil, and the second reversible adjustment unit E2 operates as a motor that receives a supply of the hydraulic oil, and rotates the rotating body when the rotation driving is carried out in the rotation driving circuit. On the other, the second reversible adjustment unit E2 operates as a pump that discharges the hydraulic oil at a high pressure by means of rotation energy of the rotating body, and the hydraulic oil at the high pressure is accumulated in a pressure accumulator Spr provided in the rotation driving circuit during a rotation deceleration. The high pressure oil accumulated in the pressure accumulator Spr is used as power to assist an engine via the reversible adjustment unit E1 depending on necessity, and the rotation energy of the rotating body during the rotation deceleration is regenerated an a result.
In the boom driving circuit, energy of the hydraulic oil discharged from the boom cylinder during a boom down operation can be accumulated via the reversible adjustment units E3 and E4 in a pressure accumulator Sph provided in the boom driving circuit, or can be supplied to the pressure accumulator Spr in the rotation driving circuit. On the other hand, the energy accumulated in the pressure accumulators Spr and Sph is converted to power via the reversible adjustment units E3, E4, and E1, thereby contributing the assist for the engine during a boom up operation.
However, the pump motor constructing the reversible adjustment unit simultaneously has the function as the hydraulic pump and the function as the hydraulic motor, and is thus expensive compared with general hydraulic pump and hydraulic motor. In the apparatus described in Patent Document 1, each of the rotation driving circuit and the boom driving circuit needs to include the multiple pump motors, the required number of the pump motors is thus large, and consequent excessive increases in cost and installation space cannot be avoided.